Percentage is simply a ratio. A ratio, a fraction whose denominator is always 100. The word percentage comes from the Latin “per centum”, wich means “by a hundred”.
For example to find percentage of blonde haired students in the a school we need to know how many blonds are there in the school and how many students are there in the school (population). Let's say there are 150 blonde students and the population of the school is exactly 1000 students. To find the ration divide 150 to 1000. The ratio is 0.15. Then multiple the ratio to 100. 0.15 × 100 = 15. The result is that 15% of the school is made of blond students.